This invention concerns a method for processing matter in a turbulent mass of particulate material.
Processes in which matter to be processed is enveloped in a heated bed of particulate material for thermal treatment have been available for a number of years.
Currently, such processes are carried out in apparatus commonly known as a fluidised bed furnace. A conventional fluidised bed furnace comprises a housing providing a processing chamber which is partially filled with refractory particles. The floor of the chamber constitutes a perforate plate and, in use, a mixture of gas and combustion air is forced up through this plate. As a result, the particles in the chamber are supported and form a turbulent mass resembling a boiling liquid. This is the "fluidised bed". Heating of the bed is effected either by combustion of the gas/air mixture below the plate before it enters the chamber, or by internal combustion of this mixture within the bed.
Matter to be processed is supplied into the bed and mixes with the heated particles by virtue of the turbulence of the bed. Such matter is thus brought into intimate contact with the heated particles and, in this environment, is effectively heated and at the same time undergoes processing. After treatment, the processed matter is extracted from the bed.
In theory, the fluidised bed just described provides an effective heat transfer mechanism which offers benefits in a variety of thermal processing systems.
In practice, however, the application of fluidised bed furnaces has been limited because of difficulties over regulating the temperatures and transfer of heat inside the bed, and problems in separating the products of certain thermal processes from the bed at the appropriate moment.
Such drawbacks have various causes:
The application of heat to the bed is at least to some extent localised. And, because the bed as a whole remains static and fluidisation is achieved by a vertical flow of the gas/air mixture within the processing chamber, the lateral movement of the bed particles is random. Consequently, the transfer of heat to some regions of the bed may be limited with the result that a non-uniform heat distribution is obtained within it.
Also, the random movement of the particles tends to give rise to uneven horizontal mixing of the particles and matter to be processed which affects the exchange of heat between the two.
Further, the currents inside the bed may actively oppose or hinder the extraction of processed matter, when solids are among the products of the process. Since these currents have no fixed direction, the solids will not naturally be made to migrate to any particular area of the processing chamber for removal, nor will they have a natural tendency to separate from the particles in the bed when they are ready for extraction. Therefore, control of the withdrawal of solid products from the chamber and their retention time within the bed tends to be uncertain.
Not only do these factors make for inefficiencies in operation but also they render the furnace unsuitable for applications where the control of temperature or retention time for solids is critical.